Covalently modification of DNA by diolepoxides metabolically formed from polycyclic aromatic hydrocarbons(PAHs) such as benzo[a]pyrene (BaP) provides a mechanism for the genotoxicity, mutagenicity, and carcinogenicity of PAHs. Using NMR and molecular modeling we have showed that the hydrocarbon of the BaP modified DNA intercalates into the DNA helix that resulted in a bending of the helical axis at the modified site. The orientation of hydrocarbon is influenced by the absolute configuration at the C10 position of the BaP. The hydrocarbon is oriented toward the 5' end of the modified strand for DNA with a 10R dA adduct and the 3' end for a 10S dA adduct. Multiple conformations, for examples, with syn and anti glycosidic torsion angles at the modified dA residue, also are observed in some of the modified DNA. We are currently investigating the effect of sequence context at the modified site on the conformation of the modified DNA. These include DNA duplexes containing -GA*C-, -CA*C- and -CA*G- sequences in the modified strand with their corresponding complimentary strands. We also extend the study to DNA containing amino alcohol adduct (BaP lacking hydroxyl groups at the C7 and C8 positions). This may provide information on the role that these hydroxyls play in the structure of the modified DNA. These studies are currently in progress. A new Mercury console that replaced a 14 years old XL console of the Varian 300 MHz NMR spectrometer has been in service over the last year. This new system significantly improves the sensitivity over the old system by about 20%, that is a saving in the experiment time by about 40%. We also replaced the old magnet shim set with a new 28 super shim set for the VXR500 NMR spectrometer early this year. The new shim set improved the field homogeneity and the spectral resolution substantially.